The differences between vertical moving beds and rotary drums

Solex Thermal Science’s vertical moving bed coolers, heaters and dryers rely on two fundamental principles for operation:

• Indirect heat exchange
• Uniform mass flow of bulk solids

The first aspect, indirect heat exchange, helps to drive significant efficiency advantages over direct heating, cooling and drying equipment (e.g.  fluidized beds, rotary drums).

Taking heating equipment as an example. A direct heat exchanger will typically pass heated air through a bed of solid media to transfer energy to the solid. However, much of the energy in the air is not transferred to the solid. Instead, it is exhausted out of the equipment as hot waste gas.

In the case of an indirect heat exchanger, energy in the heating media – whether it be steam, water or air – that is not transferred to the solid being heated is retained in the heating media circulation loop. This difference, exhausting unutilized energy versus recycling it, results in a large thermal efficiency gap between indirect and direct heat exchangers. Indirect heat exchangers are more efficient than their direct counterparts.

Indirect heat exchangers – moving bed or rotary drum

When comparing various types of indirect heat exchangers, the differences in efficiency can be more difficult to identify. The thermal efficiency of indirect systems tends to be relatively similar. This represents the lion’s share of the operating cost of a solids heat exchanger.

However, the second fundamental operating principle of Solex’s vertical moving bed heat exchangers, uniform mass flow of bulk solids, provides significant advantages.

In a typical rotary indirect drying drum, material introduced at one end of the equipment is lifted and dropped numerous times as the material moves through the drum. Keeping all of that mass continually in motion requires a significant amount of energy – not to mention the drums take up considerable floor space, measuring up to several metres in diameters and 10s of metres in length.

With Solex’s vertical moving bed heat exchangers, solids are lifted only once – typically using a bucket elevator or inclined belt. Once at the top of the unit, which typically has a plan area of two metres by two metres, gravity provides the motive force to convey the solids downward through the equipment while a small discharge device at the bottom of the heat exchanger controls the rate of throughput.

The possible energy savings in a Solex unit are significant, with a moving bed dryer requiring approximately half of the energy consumed by a comparable rotary indirect dryer.

	Indirect rotary	Vertical moving bed
Electrical fan power	10kW	10kW
Drive motor power	55kW	2kW
Bucket elevator power	-	20kW
Total power consumption	65kW	32kW
Operating hours/year	8,000 h	8,000 h
Total energy cost/year (@ $0.15/kWh)	$78,000	$38,400

Energy consumption is only one factor to consider when comparing Solex’s vertical moving bed heat exchangers with rotary indirect equivalents.

Temperature control

Solex’s vertical moving bed heat exchangers provides operators with flexibility over temperature control. Depending on the process requirement, the temperature of the heat exchange surfaces can be zoned at different temperatures, provide a gradient of temperature, or even designed to provide a specific rate of heat transfer.

Other indirect heat exchangers such as rotary steam tube systems typically provide only one zone of heating throughout the equipment.

Cleaning and maintenance

Solex’s vertical moving bed heat exchangers resemble rectangular silos featuring large access doors on either one or two sides of the equipment. These doors, which are required for fabrication, provide ready access to all internal areas of the equipment. In the event that cleaning is required, they can be opened and material can be blown from the internal surfaces of the exchanger without personnel entering the equipment.

Because material is discharged at the bottom of the exchanger, it’s a straightforward process to completely empty the equipment. In addition, the limited number and size of moving parts means there is minimal wear, routine maintenance and spare spares required to ensure continued operation.

Rotary equipment, on the other hand, can be difficult to fully discharge, while cleaning requires personnel to enter confined space access. Also, large bearings, chains and gearing require frequent preventative maintenance, and the complex fluid sealing systems required can be a source of leaks.

Operational flexibility

One area where rotary equipment tends to shine is in operational flexibility. In the event of upstream process upsets, rotary equipment is often able to process and discharge off-spec material without significant upsets in the heat exchanger itself.

In contrast, moving bed heat exchangers tend to thrive on uniformity. An influx of off-spec solids (especially sticky material) may result in fouling of the exchanger.

Discharge uniformity

In applications where uniformity of the final product is critical, Solex’s vertical moving bed heat exchangers provide an excellent solution. Each particle of solid material entering the unit will have the same residence time as every other particle, provided the discharge device has been designed correctly. As each particle has the same residence time in the exchanger, it creates highly uniform particles at the outlet.

This uniformity of residence time is more difficult to achieve in rotary drum heat exchangers, with some particles ‘short-circuiting’ and others remaining in the equipment for significant periods of time.

Size

As previously mentioned, rotary drums dominate many process plants, spanning huge lengths between concrete pillars mounted on the floor. A comparable Solex vertical moving bed heat exchanger may occupy just a few square meters of floor space.

While a Solex heat exchanger may be several stories tall, it is still often smaller overall than other indirect equipment. This is due to the use of flat heat exchange plates, compared with tubular heating elements required in most other indirect heat exchangers. For a given volume, more than double the heat exchange surface area can be found in a plate-type heat exchanger than a tube-type heat exchanger. Depending on the constraints in your process and facility, it may be more advantageous to have equipment which is shorter but larger (rotary), or smaller but taller (Solex).

Summary

There are a number of factors to consider when choosing process equipment. Even when choosing between similar types of equipment, there remain significant differences. Whether it be efficiency, maintenance, flexibility or specific process requirements, it is important to weigh all the factors involved before making a purchasing decision.

Ready to talk specifics? Contact a Solex team member today.

 

Albert Bedell, Regional Director, Asia Pacific  Albert joined Solex in 2011 and has worked in variety of roles including R&D, materials testing, applications engineering, technical service and project management prior to moving into his current role as Regional Director for the Asia Pacific region. Contact Albert